Setting a unicode character to the tab-complete suffix (in my case, ↳) prevents xchat from being able to tab-cycle through auto-complete entries.
XChat 2.8.8 on linux here
Lian Wan Situ
The problem doesn't actually have anything to do with unicode specifically.
Any setting for the completion character that takes up 3 bytes(maximum allowed size for the setting) will have the same problem.